The Science Behind Glasses and Frames
Spectacle frames are commonly made from metal or plastic. In the past, lenses used to be made from glass but glass lenses are now considered too dangerous. These days we use a special plastic typically CR-39 or polycarbonate. These materials reduce the danger of breakage and weigh less than the older type of lenses. CR-39 lenses are the most common plastic lenses due to their low weight, high scratch resistance, and low transparency for ultra violet and infrared radiation. Polycarbonate lenses are the lightest and most shatter-resistant, making them the best for impact protection and for rimless spectacles.
Some plastics also have more advantageous optical properties than glass, such as better transmission of visible light and greater absorption of ultraviolet light. Some plastics have a greater refractive index than other which means that the lenses can be lighter and thinner than ordinary materials. This is particularly useful if you are very short sighted or long sighted. Newer plastic lenses, called izon, can also correct for the higher order aberrations that naturally occur in the surface of our eye. These lenses create sharper vision and help with the halos, starbursts, and comet-tails often associated with night time driving glare.
Scratch-resistant coatings can be applied to most plastic lenses giving them similar scratch resistance to glass. Hydrophobic coatings are designed to ease cleaning. Anti-reflective coatings reduce glare, improve night vision and make the wearer's eyes more visible. Lenses can also be tinted in various colours to make sunglasses. They can even be produced to automatically darken in bright light and become clearer indoors.